Testing Web Accessibility

A great accessibility test is to put your mouse out of reach (and/or disable your trackpad) and just use the keyboard for a while.
Every time you find yourself reaching for your visual pointing device, stop and think, "Can I do this with the keyboard"?
Use Tab and Shift+Tab to navigate between controls (or grouped controls, like radios), arrow keys to navigate within controls (or control groups), and Space or Enter to select or activate. To tab out of the editor, type Ctrl+M first (to switch to Traversal Mode instead of Insert Tab Character Mode) and then type Tab or Shift+Tab. Need a context menu? Type Shift+F10. Exit a menu or dialog? Esc. Zoom in/out/restore? Ctrl +, Ctrl -, and Ctrl 0 (or Cmd +, Cmd -, Cmd 0 on Mac). List of keyboard commands in Orion? Alt+Shift+?.

Every platform and every browser has a set of keyboard shortcuts. For platform shortcuts, check out Wikipedia's Table of Keyboard Shortcuts. For browser-specific shortcuts, see the list below.
If you can't figure out how to do something with the keyboard, then maybe that something needs to be fixed.
If an important function takes too many keystrokes, maybe a key binding - or a refactoring - is needed.
If you get lost and can't figure out where the focus is, maybe you're not drawing the focus ring, or maybe it needs more contrast, or perhaps nothing has focus - which is a fairly serious bug for a keyboard-only user.

After you become more proficient with keyboard navigation and you have ironed out any serious keyboard navigation bugs, start testing with a screen reader. Learn some of the screen reader's keyboard commands.
Try listening without looking at the monitor as you navigate around your page. Does every focusable element have the label and type that you expected? Or is the screen reader saying "button" when an element you consider to be a "menu" gets focus? If you give focus to the "Create a project" text field, does the screen reader say "Create a project; Edit; type in text", or does it just say "Edit; type in text"?
Think you've got this aced? Turn off your monitor and see if you can last more than 30 seconds before turning it back on.

Screen Readers and other Assistive Technologies

Screen readers interpret what is being displayed on the screen, and then read the information aloud or send it to a braille display to assist low-vision or blind users.
Magnifiers enhance the screen with magnification and highlighting to benefit low-vision users.
Inspectors are testing tools that present semantic information to developers.

Linux

Chrome

ChromeShades inspection tool for Chrome browsers shows you visually what a screen reader user would hear

Automated Testing Tools

Automated testing tools can find low-hanging fruit, such as missing alt text on images, pages without any headings, or colors that have little to no contrast for some users. However automated testing does not take the place of clean design, user testing, and an understanding of what makes an application accessible.

Accessible Wiki Documentation

Accessible documentation includes making sure that the written text can stand alone without the images. If images are used to convey meaning, then suitable replacement text needs to be supplied.
Alternative text for an image in a wiki follows a vertical bar typed after the image file name, for example: [[Image:Orion-myimage.png|My Alternative Text]].
Exactly what the alternative text should say depends on the context. For example, here are HTML5: Techniques for providing useful text alternatives.

Make sure that links have meaningful text. Link text for a link in a wiki follows a space after the link url, for example: [http://www.google.com Google]. Avoid using words like here and here and here and more... and more... as link text because screen reader users rely on the link text to tell them what the link is for.

Tips of the Day

Orion committer Carolyn MacLeod wrote a very useful series of "tip of the day" messages that are a great place to get started with writing accessible web UIs.